As a heater module for warming the inside of a battery pack in order to adjust the temperature thereof, a heater module set forth in Patent Literature 1 is conventionally known. In Patent Literature 1, a heater module (heater body) tightly contacts the outer side surface of a casing of a battery pack which stores a number of battery modules, and the heater module is attached to the casing of the battery pack by using a heater unit casing. Also, a heat insulating sheet is placed between the surface of the heater module which faces away from the outer side surface of the casing of the battery pack, and the heater unit casing.